Technical Field
The present invention relates to a stepping motor driving device, a stepping motor driving method, a stepping motor driving program, and an electronic timepiece.
Background Art
In a stepping motor, it is necessary for the rotor to precisely rotate to each step. Thus, a stepping motor driving device determines whether or not the rotor has rotated (rotor rotation detection) by detecting back EMF (back-voltage) produced by damping when, after a driving pulse has been applied to rotate the rotor, the rotor is stopped at a prescribed step angle. A correction pulse is further applied to rotate the rotor in the case where it is determined that the rotor has not rotated.
Japanese Patent Application Laid-Open Publication No. H9-266697 can be given as an example of a motor rotation detection technique used in timepieces. This Patent Document discloses the following: “A driving pulse supplied to a step motor is subjected to chopper control such that the duty cycle of driving pulses is set to be lower in an initial stage and a final stage than in a middle stage. This makes it possible to set an effective power distribution of the driving pulses to be lower in the initial stage and the final stage and higher in the middle stage, causing the step motor to produce a torque matching the cogging torque of the step motor.
The rotor is rotated at a low speed while eliminating wasteful power consumption in the initial stage and final stage of the driving pulses, which makes it possible to reduce the amount of power consumed to drive the step motor.”
However, the technique disclosed in the stated Patent Document does not take into consideration temperature characteristics during the rotation detection. As will be described later, when the temperature changes, for example when the temperature drops, the DC resistance value will drop as well. This causes an increase in the current flowing in the motor coils. Magnetic fluxes produced by the motor coils also increase as a result, which in turn increases the rotation speed of the rotor.
Thus according to the technique disclosed in the stated Patent Document, the magnitude of the back EMF during rotation detection will change. For example, according to the technique disclosed in the stated Patent Document, a drop in the back EMF makes it difficult to accurately detect whether or not the rotor has rotated to a prescribed position, resulting in a detection error.